THE GRANDMOTHER HYPOTHESIS

Natalie Angier, New York Times News ServiceCHICAGO TRIBUNE

The Hadza people of northern Tanzania are a small group of hunter-gatherers who share a language, a culture and a distaste for gardening. Time and again, government and church agencies have sought to transmute them into full-time farmers, but the Hazda have always returned to the bush, where they subsist on wild foods like fruits, honey, tubers and game. The terrain is hard and hilly, and so is the life, but on one incomparable resource the foragers can always rely: a pack of old ladies with hearts like young horses.

As Kristen Hawkes of the University of Utah and her colleagues have found in their extensive studies of the Hadza, women in their 50s, 60s, 70s and beyond are among the most industrious members of the group. They are out in the woods for seven or eight hours a day, gathering more food than virtually any of their comrades.

When a young woman is burdened with a suckling infant and cannot fend for her family, she turns for support, not to her mate, but to a senior female relative--her mother, an aunt, an elder cousin. It is Grandma, or Grandma-proxy, who keeps the woman's other children in baobab and berries, Grandma who keeps them alive. She is not a sentiment, she is a requirement. As Hawkes, James O'Connell of the University of Utah and Nicholas Blurton Jones of the University of California at Los Angeles report in the latest issue of Current Anthropology, a nursing Hadza woman always has a postmenopausal helper. There are only about 750 Hadza, and they are contemporary hunter-gatherers, not pristine relics of prelapsarian humanity. Nevertheless, the centrality of elder women to their group's survival has thrown fresh kindling on the spirited debate over the origins and purpose of human menopause.

As doctors and women thrash out the best way to "treat" menopause, pitting the benefits of estrogen therapy to the heart and bones against the risks the hormone poses to the breast and possibly the ovaries, evolutionary scientists address the menopause mystery from a more high-flown, though no less quarrel-prone, perspective. They ask whether menopause is an ancient adaptation or a contemporary artifact. Is it the well-wrought product of natural selection, or the incidental byproduct of an unnaturally prolonged life span?

Proponents of the adaptationist camp generally see menopause as the thriftiest solution to the problem of exorbitant offspring. By this view, the ludicrous amount of time required for a mother to rear children to maturity led to the need for so-called premature reproductive senescence, an early retirement program for the ovaries. Through the mechanism of menopause, an ancestral woman theoretically was spared the risks of childbirth, and thus had a heightened chance of living long enough to see her existing children out the door.

The artifactualists insist that prehistoric women almost never survived past the age of 30, let alone long enough to experience the thrill of hot flashes. By their reckoning, menopause is a modern luxury, the result of women now outlasting an egg supply that more than sufficed for the cameo appearances that their Stone Age foremothers called lives.

"For most of our existence, we simply didn't live very long," said Alison Galloway, an anthropologist at the University of California at Santa Cruz. "Menopause happens because, through technology, we've extended our lives to the point where we run out of egg follicles. There's nothing beneficial about it."

Hawkes lends a new spin to the debate, combining elements of each camp and adding a few bold spirals of her own. She agrees with the artifactualists that menopause per se is not an adaptation--it is not the product of selective design. A woman's ovaries do not shut down "prematurely," she says. They last 40 or 45 years, the same time as the ovaries of our close relatives, chimpanzees and gorillas. In a sense, she says, women do outlive their egg supply, which is fixed before birth and cannot be added to, as men continuously generate new sperm.

On the other hand, Hawkes concurs with the adaptationists that prehistoric women very likely often survived past menopause and that they were instrumental to the survival of their families. She goes further. Only with the ascent of the grandmother, she says, were human ancestors freed to exploit new habitats, to go where no other hominid or primate had gone before, and to become the species we know so well.

"The Grandmother Hypothesis gives us a whole new way of understanding why modern humans suddenly were able to go everywhere and do everything," Hawkes said. "It may explain why we took over the planet."

The notion that menopause has adaptive value for women and their offspring is itself getting a bit long in the tooth. George C. Williams, a renowned evolutionary biologist, first proposed it in 1957 to explain the seemingly anomalous nature of human menopause. Other female primates, and even species like fin whales and elephants that live into their 80s, can continue bearing young to the bitter end, he pointed out.

Why are humans different? And why does menopause occur universally among women, and during a short window of time--at the half-century mark, give or take four years--while other depradations of age, like gray hair or presbyopia, occur gradually and randomly? Williams saw in menopause the thumbprint of natural selection and suggested that the early cessation of reproduction paradoxically enhanced a woman's reproductive "fitness" by assuring that she lived long enough to see her children bear children themselves, that she became a grandmother.

Others have picked up and expanded on the rudimentary Grandmother Hypothesis. Jared Diamond, a physiologist at the University of California at Los Angeles Medical School, has proposed that aging women, and men, were repositories of essential information in preliterate times, living libraries for their clans, able to distinguish edible from poisonous plants and to recall events of long ago that remained pertinent to survival.

Hawkes and her co-workers suggest that the extension of life past menopause was a watershed event in human prehistory. With a labor force of elder females available to help provision the young, adults were then free to colonize new territories unavailable to those primates that did not provision their weaned young, and that were thus restricted to feeding grounds where the pickings were easy enough for juvenile fingers. Suddenly humans could migrate to places where it required full adult strength and cunning to extract food. They could go wherever they pleased. After all, there were older women around to help.

Supporting this proposition are the foraging patterns of the Hadza. Very young children find much of their food themselves, but they depend on adults for half their calories, and those calories come from foods, like deeply buried tubers, that only an adult can obtain. Growing old, then, may be nothing new, and the postmenopausal years worthy of celebration and gratitude. Grandma is great, she is strong, and she baby-sits for free.